Resin powder is widely used for applying protective coatings to a variety of articles such as corrosion-resistant coatings for metal pipe or electrically insulative coatings for electrical components. The coatings may be applied by blowing powder onto articles which have been preheated above the softening point of the powder which fuses and adheres to the articles upon contact. By electrostatically charging the powder, the articles may be maintained at room temperature and ground potential so that the powder is attracted to the article. This permits the powder to be selectively removed simply by blowing or brushing it off before applying heat to fuse the remaining powder in areas where a protective coating is desired. Regardless of the technique for applying the powder, the final coatings should be of uniform thickness in all areas to be protected.
More than one year prior to the filing of the aforementioned application Ser. No. 612,639, the company to which this application is assigned began marketing for protective coatings a resin powder, the particles of which were heat-rounded or spheroidized. The spheroidizing has been carried out by allowing finely pulverized resin particles to drop through a chamber in which the particles are first heated so that at least the surface portions flow to produce rounded, almost spherical particles which are quickly cooled. Other equipment for spheroidizing pulverized resin particles is shown in U.S. Pat. Nos. 3,263,980 (Feder), 2,976,574 (Keutgen) and 3,877,918 (Cerbo).
The spheroidized resin powder which has been on the market has demonstrated significantly improved penetration into small interstices as compared to the pulverized resin powder prior to spheroidization. The improvement has been pronounced when using electrostatic coating techniques. However, when using the prior spheroidized resin powder in an electrostatically charged fluidized bed to coat a series of articles, the coatings applied to the last articles have been much thinner than those applied to the first without replenishing the bed. Then after repeatedly replenishing the powder, it has been necessary to clean out the bed, discarding the remaining powder in order to avoid undesirably rough coatings.
Although some of the particles of the prior spheroidized resin powder did not fuse in the spheroidizing process, it was believed that the highest possible degree of heat-rounding was desirable. A relatively high percentage of the particles present when it became necessary to clean out the bed were not rounded, although there is no indication that anyone had examined them for shape. In any event, applicants have found that when resin powder containing very high percentages of rounded particles is used in an electrostatically charged fluidized bed, the coatings tend to have undesirable crater-like depressions.